Identify the bike stem system
First determine whether your bicycle uses a threadless or quill-style stem. They attach differently and are not direct substitutes. Check the bicycle documentation or ask a mechanic if the system is unclear.
Measure both clamp diameters
The stem must match the steering tube interface and the handlebar's clamping area. Measure the actual clamping sections, or use dimensions printed on the existing parts. Do not assume a diameter from the bicycle's age, brand, or wheel size.
Measure stem length and angle
Stem length is normally measured from the steering-axis center to the handlebar-clamp center. Angle and orientation change bar height and reach. Even a compatible stem can noticeably change handling, so compare the new dimensions with the existing one before fitting it.
Treat installation as safety-critical
Use the manufacturer's fastener sequence and torque values. Confirm that all minimum insertion, spacer, and clamping requirements are satisfied before riding.